Phylum Arthropoda - The Animals With Jointed Feet
Category : 11th Class
(Gk. Arthron = joint; Podos = foot)
Brief History : Aristotle described a few crabs and other arthropods. Linnaeus included all such animals in his group “Insecta”. Lamarck divided this group into three classes – Crustacea, Hexapoda and Arachnida. Finally, Von seibold (1845) established the phylum Arthropoda for these animals.
General characters
(1) Occur widely on land, in air, and in all sorts of water, from snowy tops of high mountains to the depths of ocean. Many are parasites of other animals and plants. Hence, the phylum is of great economic importance.
(2) Bilateral, triploblastic, body segmented and also divided into head, thorax and abdomen. Segmentation marked only externally; number of segments or somites fixed and each has its separate exoskeleton of thick and hard, chitinous cuticle secreted by epidermis of body wall. Head somites always fused.
(3) Each segment basically bears a pair of lateral jointed appendages adapted for food ingestion, locomotion, respiration, copulation, etc.
(4) Muscular system well-developed; muscle fibres always striated.
(5) Digestive tract complete. Most head appendages from mouth parts with lateral jaws for chewing or sucking. Anus terminal.
(6) Coelom reduced to small cavities in excretory and reproductive organs; replaced elsewhere by blood sinuses which merge together to form a large perivisceral cavity – the haemocoel–around viscera. Sinuses form an “open blood vascular system” filled with haemolymph which may contain haemocyanin. Haemocoel communicates with a long tubular and pulsatile, mid–dorsal heart.
(7) Respiration by gills (aquatic forms), or trachea or book lungs (terrestrial forms); by diffusion through body surface in some.
(8) Excretion by coelomoducts or specialized green or coxal glands, or by malpighian tubules. Excretory product is uric acid.
(9) Nervous system basically similar to the typical annelid plan; head with a brain-ring which is connected to a double ventral nerve cord, having paired segmental ganglia which represent true metamerism. Well–developed sensory organs of various types.
(10) Cilia completely absent. Muscles mostly striated and capable of rapid contraction.
(11) Sexes mostly separate with sexual dimorphism. Paired reproductive organs and ducts.
(12) Fertilization typically internal, in female's body. Eggs megalecithal. Oviparous or viviparous
(13) Life–cycle includes one or more larval stages that metamorphose into adults.
Classification of Arthropoda : On the basis of body shape, degree of segmentation and regionation, and presence or absence of certain appendages (antennae, mandibles and chelicerae), phylum Arthropods is divided into four subphyla; Biggest phylum in regard to the number of species is Arthropoda.
Subphylum (I) Onychophora (Gr. Oychos = claw, phoros = bearing)
(1) Terrestrial walking worms.
(2) Body cylindrical with indistinct extenral segmentation.
(3) Unjointed 14-43 pairs of legs.
(4) Head not distinct, Oviparous/Viviparous
(5) A pair of eyes, short antennae and blunt oral papillae.
(6) Excretory organs are metanephridia segmentally arranged.
(7) A living connecting link forming a transitional link between Annelida and Arthropoda.
Example : Peripatus, Ophisthopatus, Ooperipatus, etc.
Subphylum (II) Trilobitomorpha (Gr. TRIA = Three; LOBOS = lobe; MORPHE = form)
(1) Most primitive, extinct, marine arthropods of Cambrian to Permian rocks.
(2) 10 to 675 mm. Long body covered by a hard segmented shell; distinct head of four fused somites bearing a pair of antennae, four pairs of appendages and often a pair of eyes.
(3) Trunk divided, by two longitudinal furrows, into 3 lobes.
(4) Abdominal region of 2 to 29 somites and a fused caudal plate or pygidium.
(5) Each segment, except the last one, bears a pair of biramous jointed appendages.
Example – Triarthrus, Dalmenites.
Subphylum (III) Chelicerata (Gr. CHELA = Claw; CEROS = Horn; ATA = Group)
(1) Mostly terrestrial, free–living and small–sized.
(2) Body distinguished into head, thorax and abdomen (= opisthosoma). Head and thorax fused to form a cephalothorax or prosoma.
(3) Cephalothorax with eyes and six pairs of appendages – One pair of clawed and jointed chelicerae in place of mandibles, one pair of pedipalps, and four pairs of walking legs. Antennae absent. Abdomen with or without appendages, but distinguished into a large and broader mesosoma, a small metasoma and a long and narrow, tail–like telson.
(4) Respiration by gills book–lungs or tracheae.
(5) Excretion by malpighian tubules or coxal glands, or both.
(6) Sexes mostly separate; females oviparous; development direct or through a larval stage. Divided into three classes on the basis of respiratory organs
Class 1. Merostoma
(1) They are Marine.
(2) Respiration by gills.
(3) Cephalothorax with lateral compound eyes median simple eyes and six pairs of usual appendages.
(4) Abdomen with 5 to 6 pairs of gill–bearing appendages.
(5) Hind end forms a long bayonet–like telson.
Example– Limulus (The king-crab).
Class 2. Arachnida
(1) Mostly terrestrial; spiders, scorpions, mites, ticks, etc.
(2) Respiration by book–lungs or trachea.
(3) Eyes simple.
(4) Abdomen without appendages.
(5) Many with poison glands and poison fangs, jaws of stings.
(6) No gills.
(7) Life–cyle without metamorphosis.
Examples– Palamnaeus (scorpion), Lycosa, mites, ticks.
Class 3. Pycnogonida or Pentapoda
(1) Small–sized marine sea–spiders.
(2) Cephalothorax 3–segmented; forms major part of body; abdomen vestigial.
(3) Suctorial mouth on top of a long proboscis.
(4) Head usually with 4 pairs of appendages and 4 eyes.
(5) 5, 6 or 12 pairs of long walking legs.
(6) No special respiratory and excretory organs.
(7) Unisexual; females oviparous. Eggs carried by males.
Example – Nymphon.
Subphylum (IV) Mandibulata or Antennata (L.MANDIBULA = Mandible; ATA =group)
(1) Body divided into head and trunk, or head, thorax and abdomen.
(2) Segmentation distinct.
(3) 1 or 2 pairs of antennae, 1 pair of mandibles in place of chelicerae, one or more pairs of maxillae and 3 or more pairs of walking legs.
(4) Eyes mostly compound.
(5) Respiration by gills or trachea.
(6) Excretion by malpighian tubules or antennal glands.
(7) Unisexual; life cycle usually with larval forms. Divided into six classes.
Class 1. Crustacea
(1) Mostly aquatic.
(2) Body divided into cephalothroax and abdomen.
(3) Dorsally, cephalothorax covered by a thick exoskeletal carapace.
(4) Head of 5 segments, with 2 pairs of antennae, one pair of mandibles and 2 pairs of maxillae; thorax of 2 to 60 distinct or variously fused somites; abdominal somites usually distinct with a posterior telson.
(5) Appendages mostly biramous.
(6) Respiration through body surface or by gills.
(7) Excretion by special coxal glands in antennae or maxillae.
(8) Mostly unisexual; genital ducts and pores paired; females oviparous.
(9) Life-cycle usually with larval forms.
Examples – Palaemon, Cancer, Cyclops, Astacus, Sacculina, Cypris, Daphnia, etc.
• Tiny crustaceans such as Daphnia and cyclops act as zooplankton which form important link in the food chain in water.
Class 2. Insecta
(1) Aquatic, terrestrial or aerial.
(2) Body divided into head, thorax and abdomen.
(3) Segments 6 in head, 3 in thorax and 11 or less in abdomen.
(4) Legs typically 3 pairs (Hexapoda); aerial forms with one or two pairs of wings.
(5) Head with 1 pair of large, compound eyes, 1 pair of antennae and variously modified mouth–parts.
(6) Respiration by branched tracheae.
(7) Excretion by specialized malpighian tubules.
(8) Unisexual; females oviparous.
(9) Life-cycle simple or complicated.
Examples – Periplaneta, Musca, Mosquitoes, locusts, butterflies, bees, wasps, termites, silverfish, beetles, etc.
Class 3. Diplopoda
(1) Terrestrial.
(2) Body long, cylindrical, worm–like.
(3) 5-segmented head with 1 pair each of short antennae, mandibles and maxillae; 2 groups of simple eyes.
(4) Thorax of 4 segments, each except the first with a pair of joined legs.
(5) Abdomen of 9 to 100 or more segments, but each apparent segment formed by fusion of two and, hence, bears 2 pairs of legs, spiracles, ostia and nerve ganglia.
(6) Respiration by tracheae.
(7) Excretion by malpighian tubules.
(8) Unisexual; gonad single; females oviparous.
Example – Thyroglutus (millipede)
Class 4. Chilopods
(1) Terrestrial.
(2) Body long, worm–like, somewhat dorso–ventrally flattened and divided into head and trunk.
(3) Segments 15 to 181; not fused in pairs; each with a single pair of legs; first pair of legs claw like and each contains a poison gland.
(4) Head with a air each of long antennae and mandibles, and 2 pairs of maxillae.
(5) Respiration by tracheae.
(6) Unisexual; females oviparous or viviparous. Genital openings mid ventral on last but one segment.
(7) Excretion by malpighian tubules.
Example – Scolopendra (centipede).
Class 5. Symphyla
(1) Terrestrial.
(2) Body upto 6 mm. Long; divided into head and trunk.
(3) Head like that of insects, but without eyes.
(4) Trunk of 15 to 22 somites; bears 10 to 12 pairs of legs.
(5) Genital pores mid ventral between legs of 4th pair.
Example – Scutigerella (the garden centipede).
Class 6. Pauropoda
(1) Terrestrial.
(2) Minute, soft and cylindrical, worm like body divisible into head and trunk.
(3) Head with one pair each of branched antennae and unbranched mandibles and maxillae; no eyes.
(4) Trunk of 11 or 12 somites which are dorsally fused in pairs.
(5) Legs 9 to 10 pairs.
(6) Genital pores ventral on 3rd trunk segment.
Example – Pauropus.
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